Ten years or more have passed since a lithium ion secondary battery was manufactured as a commercial product for the first time. Rapid progress of portable devices have been made and these portable devices have spread since then. It is needless to say that the characteristics, such as high output and high energy density of the lithium ion secondary battery largely contributes to this background. LiCoO2 is generally used as the positive electrode active material of this lithium ion secondary battery. LiCoO2 exhibits a charge-discharge potential in the 4 V class in combination with a metal Li counter electrode, is synthesized relatively easily and is allowed to have a capacity of about 150 to 160 mAh/g. Therefore, this lithium battery is advantageous to constitute a battery having high energy density by using LiCoO2. However, Co that is a structural element of LiCoO2 is expensive. Also, this battery is not necessarily suitable to large-scale batteries used in the applications of HEV (hybrid car) assumed to be driven for a long term like 10 to 20 years in view of long-term reliability and easiness of mass-production.
In HEV applications, it is desired for battery to suppress a rise in battery resistance caused, particularly, by cycles and storage and improve high rate charge-discharge characteristics besides excellent charge-discharge cycle characteristics at high temperatures and excellent capacity retention properties at high temperatures which are the characteristics required for conventional batteries. Under this situation, new positive electrode materials that substitute for LiCoO2 are sought in the fields where high rate characteristics and long-term reliability such as in applications of HEV are required and there are strict demands for low cost.
Studies are made as to practical use of LiNiO2 type materials having a bedded salt structure and LiMn2O4 having a spinel structure as materials for small batteries in portable device applications. Among these materials, LiNiO2 type materials have a charge-discharge capacity as large as 170 to 200 mAh/g though operating voltage is slightly lower than that of LiCoO2, and it is therefore possible to decrease the cost per capacity. However, in order to use LiNiO2 type materials safely, various suppression must be added, the material not necessarily reach the stage where it is regarded as a most promising one of next positive electrode active materials.
On the other hand, Li-containing complex oxides that are represented by LiMn2O4 and have a cubic system spinel structure are superior in high rate charge-discharge characteristics due to its crystal structure having a three-dimensional diffusing path of Li and are also highly safe and inexpensive due to the stability of Mn4+. These Li-containing complex oxides are therefore expected as positive electrode active materials suitable in HEV applications.
However, LiMn2O4 is changed in characteristic more significantly under a high-temperature circumstance than other layer oxides and is therefore deteriorated in capacity with increased temperature due to charge-discharge cycles and storage.
The reason why LiMn2O4 is inferior to LiCoO2 in charge-discharge cycle characteristics is usually said to be the Jahn-Teller strain caused by a plus trivalent Mn ion or the elution of Mn in an electrolytic solution from a lithium manganate crystal. In view of this situation, technologies of making a composition of excess Li, specifically, Li1+xMn2-xO4 and a method in which the Mn site is replaced with other elements and particularly with Cr are investigated (Japanese Patent Application Laid-Open (JP-A) Nos. 6-187993, 5-36412).
These technologies are oxygen octahedron as center on the Mn ion for making firm that basically bring the Mn value number balance in the manganese acid lithium close to +4 values. Therefore, the improvement in charge-discharge cycle characteristics which are made by using these technologies is confirmed experimentally. However, the improvement is not enough to meet the requirements for power storage and power source for electromobiles.
Also, methods in which the surface of LiMn2O4 is covered with other materials are investigated separately from the approach to the technologies by using the substitution with Li and other transition metal elements. Patent JP-A No. 2002-68745 discloses technologies for covering the surface of Al-substitution type LiMn2-yAlyO4 with Li-excess type Li1+xMn2-xO4.
However, the technologies for covering the surface of LiMn2O4 are measures taken to select a chemically, thermally or electrochemically stable material and therefore, serve to hinder Li from going in or out through the interface between the positive active material and the electrolytic solution as viewed from a new angle. For this, these technologies are unnecessarily measures suitable to attain high-rate discharge characteristics.
In view of this situation, as a further measures taken to approach to the subject, a trial is made to improve charge-discharge cycle characteristics or capacity retention properties by adding other materials in a battery or an electrode instead of improving the crystal structure itself of lithium manganate and covering the surface of a crystal (JP-A No. 2001-506052). On the assumption that the reason of the deterioration of lithium manganate is the acid produced in an electrolytic solution and phenomena caused by the acid such as dissolution of lithium manganate and decomposition of an electrolytic solution and supporting salt, the technologies described in patent reference intends consciously to prevent a deterioration in battery characteristics by suppressing the above phenomena and the like. Though this method has a certain effect on the improvement in battery characteristics in the case of assuming that it is applied to a power source for small portable devices, this method is unsatisfactory in light of suppression on a change in the internal resistance of a battery when assuming that it is used in HEV applications. Therefore a further improvement was needed in the battery of HEV applications.
On the other hand, a method using colorimetry is proposed when the quality of a battery is controlled (JP-A No. 8-50900). The characteristics of the color of a positive electrode active material is considered to reflect the characteristics such as its structure. However, in JP-A No. 8-50900, a calorimetric value is used only for discriminating between a nondefective and a defective, and this method is not based on the idea intending to improve the characteristics of a battery by utilizing the color characteristics of a product from the stage of a product design.